Referring to FIG. 1, a color CRT 10 generally comprises an evacuated glass envelope consisting of a panel 12, a funnel 13 sealed to the panel 12 and a tubular neck 14 connected by the funnel 13, an electron gun 11 centrally mounted within the neck 14, and a shadow mask 16 removably mounted to an inner sidewall of the panel 12. A three color phosphor screen is formed on the inner surface of a display window or faceplate 18 of the panel 12.
The electron gun 11 generates three electron beams 19a or 19b, said beams being directed along convergent paths to the shadow mask 16 by means of several lenses of the gun and a high positive voltage applied through an anode button 15 and being deflected by a deflection yoke 17 so as to scan over the screen 20 passing through apertures or slits 16a formed in the shadow mask 16.
In the color CRT 10, the phosphor screen 20, which is formed on the inner surface of the faceplate 18, comprises an array of three phosphor elements R, G and B of three different emission colors arranged in a cyclic order of a predetermined structure of multiple-stripe or multiple-dot shape and a matrix of light-absorptive material(black matrix) 21 surrounding the phosphor elements R, G and B, as shown in FIG. 2.
A thin film of aluminum 22 or electro-conductive layer, overlying the screen 20 in order to provide a means for applying the uniform potential applied through the anode button 15 to the screen 20, increases the brightness of the phosphor screen, prevents ions from damaging the phosphor screen and prevents the potential of the phosphor screen from decreasing. And also, a resin film 22' such as lacquer is applied to the phosphor screen 20 before forming the aluminum thin film 22, so as to enhance the flatness and reflectivity of the aluminum thin film 22, then being baked and driven off for a CRT's longer life after forming the aluminum film 22.
In a photolithographic wet process, which is well known as a prior art process for forming the phosphor screen, a slurry of a photosensitive binder and phosphor particles is coated on the inner surface of the faceplate. It does not meet the higher resolution demands and requires a lot of complicated processing steps and a lot of manufacturing equipments with the use of a large quantity of clean water, thereby necessitating higher cost in manufacturing the phosphor screen. In addition, it discharges a large quantity of effluent such as waste water, phosphor elements, 6th chrome sensitizer, etc.
To solve or alleviate the above problems, an improved process of electrophotographically manufacturing the screen utilizing dry-powdered phosphor particles is developed.
U.S. Pat. No. 4,921,767, issued to Datta at al. on May 1, 1990, discloses the improved method of electrophotographically manufacturing the phosphor screen assembly using dry-powdered phosphor particles through a series of steps as is briefly explained in the following.
The method comprises the steps of: (a) coating said inner surface of the panel with a volatilizable conductive layer; (b) overcoating said conductive layer with a volatilizable photoconductive layer, the volatilizable photoconductive layer containing a material responsive to visible light; (c) establishing a substantially uniform electrostatic charge on said photoconductive layer; (d) exposing selected areas of the volatilizable photoconductive layer to visible light, so as to selectively discharge the electrostatic charges from the volatilizable photoconductive layer; (e) applying a triboelectrically charged first color emitting phosphor onto said exposed, selected areas of the photoconductive layer; (f) fixing said U first color emitting phosphor onto said photoconductive layer; (g) repeating steps (c), (d), (e), and (f), consecutively, for triboelectrically charged second and third color emitting phosphors to form a luminescent screen comprising picture elements of triads of color-emitting phosphors; (h) aluminizing said luminescent screen; and (i) baking said faceplate panel to remove the volatilizable constituents from said luminescent screen to form said luminescent screen assembly. The same process of the above steps can be repeated also for the black matrix particles before or after the three different phosphor particles are formed, thereby forming a screen array 20 of light-absorptive material 21 and three phosphor elements R, G and B in FIG. 2.
The conventional method of electrophotographically manufacturing the phosphor screen assembly using dry-powdered phosphor particles as described above has one problem that it requires dark environment during all the steps until the fixing step after the photoconductive layer is formed, because the photoconductive layer is sensitive to the visual light. Also, the fixing step is still necessary even after the developing step.
To overcome this problem, the applicant proposed a method of forming the photo-conductive layer using a photo-conductive solution responsive to the ultraviolet rays.
The solution for the photo-conductive layer 134 responsive to the ultraviolet rays, for example, may contain: an electron donor material, such as about 0.01 to 1 percent by weight of bis-1,4-dimethylphenyl (-1,4-diphenyl (butatriene)) or 2 to 5 percent by weight of tetraphenylethylene (TPE); an electron acceptor material, such as about 0.01 to 1 percent by weight of at least one of trinitrofluorenone (TNF) and ethylanthraquinone (EAQ); a polymeric binder, such as 1 to 30 percent by weight polystyrene (PS); and a solvent such as the remaining percent by weight of toluene or xylene.
Meanwhile, Japan Patent Laid-open publication No. PYUNG 08-236036, published on Sep. 13, 1996, discloses "DISPLAY SURFACE AND ITS MANUFACTURING METHOD", wherein, as shown in FIG. 5, both an inner-surface anti-reflection film 1 formed over the whole area of a faceplate 18, and a pigment-particle layer 1 formed only beneath a black pigment layer 2 contain silicon oxide particles, have an interference effect of a reflection light of a incident outer light and an absorption effect of a dispersion light of the incident outer light, thereby sufficiently preventing reflection of the incident outer light and improving contrast. Accordingly, the color purity and brightness of the faceplate 18 with higher transmissivity of light can be improved.
The inner-surface anti-reflection film or the pigment-particle layer 1 is manufactured by a conventional photolithographic wet process as shown in FIGS. 6a to 6d. That is, a resist pattern 1' as shown in FIG. 6a can be obtained by the steps of coating a photoresist liquid on the faceplate 18, exposing the photoresist coating through a shadowmask to a high pressurized mercury lamp and developing the exposed photoresist coating. Then, as shown in FIGS. 6b and 6c, the pigment-particle layer 1 and the black pigment layer 2 in order are coated on the resist pattern 1', and a resist-dissolving liquid containing a sulfamine acid of 10% is applied thereto, thus the resist pattern 1' is removed and the black pigment layer 2 of a matrix pattern with the pigment-particle layer 1 beneath the matrix pattern, as shown in FIG. 5, is manufactured.
Also, a blue color pigment layer, a green color pigment layer and a red color pigment layer can be formed beneath a blue color-emitting phosphor, a green color-emitting phosphor and a red color-emitting phosphor, respectively, by the above-mentioned method, thus a screen structure with a color filter formed beneath the phosphor can be obtained.
Furthermore, Japan Patent Laid-open publication No. PYUNG 08-106859, published on Apr. 23, 1996, discloses "COLOR CATHODE RAY TUBE", wherein, as shown in FIG. 7, a green color filter layer 3, a blue color filter layer 4 and a red color filter layer 5 are formed, respectively, by the above-mentioned method, and only a white color-emitting phosphor 6 is formed on the filter layers, thereby improving brightness of 10%, decreasing reflexibility of 10% and increasing productivity as compared with the prior art phosphor layer with a color filter.
However, since the methods disclosed in the aforementioned publications use a photolithographic wet process for manufacturing a filter layer, which is well known as a prior art process for forming the phosphor screen, they do not meet the higher resolution demands and require a lot of complicated processing steps and a lot of manufacturing equipments with the use of a large quantity of clean water, thereby necessitating higher cost in manufacturing the phosphor screen. In addition, a large quantity of effluent such as waste water is discharged.
Moreover, in the photolithographic wet process for forming the filter layer, conglomeration among the particles is caused due to particle dispersion and cohesion, thereby making the thickness of the filter layer un-uniform and the brightness of the phosphor screen partially different over the whole areas of the faceplate.
Furthermore, in the aforementioned dry-electrophotographically manufacturing method of a screen, the red color-emitting phosphor comprises Y.sub.2 O.sub.2 S:Eu/Fe.sub.2 O.sub.3, the blue color-emitting phosphor ZnS:Ag/CoO.nAl.sub.2 O.sub.3, the green color-emitting phosphor ZnS:Cu,Al. Thus, only Fe.sub.2 O.sub.3 and CoO.nAl.sub.2 O.sub.3 as red and blue pigments put in colors to the phosphors and therefore, color purity and brightness are not improved.
Therefore, the present invention has been made to overcome the above described problems, and thereby it is an object of the present invention to provide a method of electrophotographically manufacturing a viewing screen including a filter layer of pigment particles for a cathode-ray tube(CRT), which can improve its color purity and brightness in a large way.